Explicitly set rpc_backend to kombu when using rabbitmq

On centos systems, /usr/share/neutron/neutron-dist.conf sets:

rpc_backend = neutron.openstack.common.rpc.impl_qpid

In order to set this correctly when using rabbit, we need to explicitly
set it in neutron.conf to override the setting in the dist file

- explicitly set rpc_backend when rabbitmq used
- add wrap around commented out zmq section

Change-Id: Ifea0ecc7ec50f7a0938b1bdd19f9da73dcb42c1e
Implements: blueprint centos-rhel-for-network
Merge "Explicitly set rpc_backend to kombu when using rabbitmq"